The invention relates to a hydrometallurgical recovery of gold and silver by direct oxidizing sulphuric acid-digestion of ore-concentrates, particularly arsenopyrite-concentrates (FeAsS.sub.2) containing carbonaceous materials, with a silicate gangue, and/or a silicate and pyrite gangue. In the recovery, arsenic and iron are substantially fully solubilized and the noble metals are substantially quantitatively enriched together with the carbon of the carbonaceous materials in the silicate residue. After decarbonization of the residue gold and silver can be recovered substantially without losses due to adsorption by cyanide leaching and subsequent precipitation.
The normal method to recover gold and silver from arsenopyrites is to concentrate it by flotation. Arsenopyrites always contain silicates as gangue and depending on the type of ore, pyrite and carbonaceous materials such as graphite. Because the roasting process used nowadays for destroying sulphide matrix is thermally uncontrollable when carbonaceous materials are present, it is necessary to depress the carbonaceous materials during flotation to produce carbon-free aresenopyrite-concentrates. This works only partially and is out of the question when the carbon contains absorbed noble metals.
Arsenopyrites decompose in a temperature range between 500 and 800.degree. C. To liberate the content of gaseous arsenic as As.sub.2 O.sub.3, the arsenic and the arsenic sulphide in the gas phase have to be fully oxidized. Therefore, a low oxygen-pressure and a high SO.sub.2 -partial pressure are necessary in the roasting zone. An oxygen-pressure which is too high will produce metal-arsenates. The overall equation of the roasting process of arsenopyrite is: EQU 4 FeAsS+10 O.sub.2 .fwdarw.2 Fe.sub.2 O.sub.3 +2As.sub.2 O.sub.3 +4 SO.sub.2 ( 1)
This technique has many disadvantages. First, the unavoidable emission of SO.sub.2 and As.sub.2 O.sub.3 means an unacceptable environmental pollution. On the other hand, the loss of gold due to dust discharge is (dependent on the temperature of roasting) more than 30%. At 802.degree. C., a loss of gold of 33.7% has to be expected (see also: Ullmanns Enzyklopadie der Technischen Chemie, Verlag Chemie, Weinheim/Bergstr., 1974). There will be an additional loss of noble metals in the following cyanidation due to non-complete roasting because of arsenate- or ferroarsenate over production, and due to inclusion during the sintering of the resulting hematite (Fe.sub.2 O.sub.3)
Many attempts have been made to replace the pyrometallurgical step of roasting arsenopyrite-concentrates by hydrometallurgical processes. One proposal is the oxidizing pressure-leaching of arsenopyrites in an autoclave using NaOH, an oxygen-pressure of 10 bar, and a temperature of 100.degree. C. During this process, arsenic is transformed into water soluble Na.sub.3 AsO.sub.4 and the sulphide is oxidized to sulphate. The leaching residue consists mainly of Fe.sub.2 O.sub.3 and the noble metals (Pawlek, F., Metallhuttenkunde, Verlag Walter de Gruyter, Berlin, N.Y., 1983, p. 639).
This process has the disadvantage that the silicate gangue will be co-leached in the main, so that there will be problems with filtration of the solid/liquid separation due to gel formation. Additionally, the essentially amorphous resulting Fe.sub.2 O.sub.3 has very good solubility, so that high reagent costs have to be expected for the anticipated dissolution of the metals in cholorine gas or cyanide solution.
The oxidative, acidic pressure digestion of arsenopyrites is generally not possible on the conditions known for alkaline digestion. On the one hand, the reaction rate is too slow, and on the other hand, a long reaction time causes hydrolysis with the formation of insoluble arsenates and the alkaline sulphates, which make the recovery of noble metals by cyanidation in the presence of carbonaceous materials impossible by adsorption (Gerlach, J. and others: EinfluB des Gitteraufbaus von Metallverbindungen auf ihre Laugbarkeit, Erzmetall, 1972, p. 450).
A new process by Stearns Catalytic Ltd. and Arseno Processing Ltd. (Gold recovery from arsenopyrite by the Arseno Process, Western Miner., March 1983, p. 21) discloses that the oxidizing, acidic pressure-digestion of pyrite-free arsenopyrite-concentrates is possible at temperatures of 100.degree. C., when a catalyst is used. The conditions of reaction are an oxygen-pressure of 7 bar and a reaction time of 15 min. Although this method may be the best way of processing pyrite-free arsenopyrite-concentrates which contain gold, it has the following disadvantages:
1. The process depends on the use of a catalyst, which cannot be regenerated.
2. Sulphides will be oxidized only to elementary sulphur, which will of necessity mix with the silicate-gold residue during the solid-liquid-separation. During the following oxidizing cyanidation in a basic medium, the sulphur reacts with the oxygen to form thiosulphate, polysulphate, sulphate and sulphite. Less than 0.05 ppm of of sulphite (S.sup.2) will reduce the recovery considerably (Adamson, R. I., Gold Metallurgy in South Africa, Cape + Transvaal Printers Ltd., 1972).
3. The carbonaceous materials and the gold are concentrated in the silicate residue. It is alleged that the carbonaceous materials are passivated during the process, so there will be no losses of gold due to adsorption during the following cyanidation. However, when the carbon is passivated, the amount of noble metal occluded in the carbon-particles is not recoverable by cyanidation, so that there will be losses in output.
4. Only when no pyrite is present, is it possible to keep the stated reaction conditions (100.degree. C., 7 bar, 15 min.); at 100.degree. C. and an oxygen-pressure of 8 bar, a maximum 20% of the total pyrite can be dissolved in 15 min. (Hahne, H.: Beitrag zur Drucklaugung von Eisensulfiden, Diss. TU Berlin, 1964). The removal of pyrite from arsenopyrite-concentrates requires another process-step (flotation). However, this is only possible when the pyrites are free from gold, which is mostly not the case.
5. Silver is found in the gold-containing residue as well as in the arsenic-iron-solution. The dissolved part is thus not recoverable and represents a heavy loss.
Accordingly, it is an object of the invention to provide a hydrometallurgical process for the recovery of gold and silver as well as a rich gold and rich silver containing, iron-, arsenic-, and carbon-free silicate concentrate, from ore concentrates, particularly from arsenopyrite concentrates, and, more particularly, from pyrite-containing arsenopyrite concentrates, which contain carbonaceous substances as well as silicates.
It is also an object of the invention to provide a process which enables a substantially quantitative yield of gold and silver containing, iron-, arsenic-, and carbon-free silicate concentrate under the most economical process conditions while largely avoiding environmental pollution.